Wind has been used for a long time as a source of power and in recent years it has become very common to use the wind for producing electrical power. In order to do so, the power in the wind is captured by a set of blades (normally two or three) of a wind power plant. The wind captured by the blades causes a shaft connected to the blades to rotate. The shaft is connected to a rotor of a generator, which hence rotates at the same speed as the shaft, or at a multiple of the speed of the shaft in case the rotor is connected to the shaft via a gearbox. The generator then converts the mechanical power provided by the wind into electrical power for delivery to a grid.
In order to optimize the efficiency of a wind turbine generator, it is preferred to use a variable speed generator, wherein the speed of the blades and hence the speed of the shaft depend on the wind speed. This implies that an optimum operating point for the WTG at various wind speeds must be established. This is done by controlling the torque and active (real) power delivered by the generator.
The primary purpose of a WTG is to deliver active power. Active power is the component of total, or apparent, electric power that performs work and is measured in watts. The control system in a WTG will control the active power drawn from the WTG in order to track the optimum speed operating point for the WTG by using either torque control or power control.
When power control is used, a power command based on an estimated power in the wind is fed to the control system. This commanded value is compared to the actual WTG output power and the difference is controlled.
When torque control is used, a torque command based on a torque available from the shaft is fed to the control system. This commanded value is compared to the actual generator torque and the difference is controlled.
Reactive power, measured in volt-amperes, establishes and sustains the electric and magnetic fields of alternating current machines. The apparent power, measured in volt-amperes, is the vector sum of the real and reactive power. Control systems of modern WTG may control both active and reactive power to the grid.
A first type of control systems for WTGs relate to independent control of (normally) three 120° spatially displaced sinusoidal voltages from the three stator phases of the generator. The generation of the sine waves is based on the properties of the generator, i.e. an equivalent model for the generator when operating in its steady state is derived from the electrical and mechanical characteristics of the generator wherein the control system is designed based on the type of generator used (e.g. asynchronous or synchronous).
The generation of one of the sine waves in the three phase system is normally performed independently of the other sine waves, i.e. this type of control systems operate as three separate single phase system controls rather than one common control of a three phase system. This fact result in that any imbalance in the three phase system or any interaction between the phases will not be considered in this type of control. Moreover, it is evident that the generator model will only be valid during steady state operation of the generator. During transient operation of the generator (start, stop, load changes etc) the control will hence allow high peak voltage and current transients. This result in a decreased power conversion efficiency as well as a need to oversize the electrical components of the WTG system in order to cope with transient surge currents and voltages.
In order to overcome the drawbacks of the above control structure, an alternative control structure generally named Field Orientated Control (FOC) have been introduced. The main idea behind FOC is to control the stator currents of the generator by using a vector representation of the currents. More specifically, FOC is based on coordinate transformations which transform a three phase time and speed dependent system into a two coordinate time invariant system.
The advantage of performing a transformation from a three phase stationary coordinate system to a rotating coordinate system is that the control of the generator may be done by controlling DC quantities and the response to transients is improved over that achievable with independent control over the three phase.
The transformation for FOC is performed in two steps: 1) transformation from the three phase abc stationary coordinate system to a two phase so called αβ stationary coordinate system (known as Clarke transformation), and 2) transformation from the αβ stationary coordinate system to a dq rotating coordinate system (known as Park transformation). More specifically, the transformation from the natural abc reference frame to the synchronous dq reference frame is obtained by the equations
            [                                                  α              u                                                          β              u                                                          0              u                                          ]        =                  [                                                            a                u                                                                    b                u                                                                    c                u                                                    ]            ⁢                        2          3                ⁡                  [                                                    1                                            0                                                              1                  2                                                                                                      -                                      1                    2                                                                                                                    3                                    2                                                                              1                  2                                                                                                      -                                      1                    2                                                                                                -                                                            3                                        2                                                                                                1                  2                                                              ]                                and      ⁢                          [                                                  d              u                                                          q              u                                                          0              u                                          ]        =                  [                                                            α                u                                                                    β                u                                                                    0                u                                                    ]            ⁡              [                                                            cos                ⁢                                                                  ⁢                θ                                                                                      -                  sin                                ⁢                                                                  ⁢                θ                                                    0                                                                          sin                ⁢                                                                  ⁢                θ                                                                    cos                ⁢                                                                  ⁢                θ                                                    0                                                          0                                      0                                      1                                      ]            which gives
      [                                        d            u                                                q            u                                                0            u                                ]    =            [                                                  a              u                                                          b              u                                                          c              u                                          ]        ⁢                  2        3            ⁡              [                                                            cos                ⁢                                                                  ⁢                θ                                                                                      -                  sin                                ⁢                                                                  ⁢                θ                                                                    1                2                                                                                        cos                (                                                                  ⁢                                  θ                  -                                                            2                      ⁢                      π                                        3                                                  )                                                                    -                                  sin                  (                                                                          ⁢                                      θ                    -                                                                  2                        ⁢                        π                                            3                                                        )                                                                                    1                2                                                                                        cos                (                                                                  ⁢                                  θ                  +                                                            2                      ⁢                      π                                        3                                                  )                                                                    -                                  sin                  (                                                                          ⁢                                      θ                    +                                                                  2                        ⁢                        π                                            3                                                        )                                                                                    1                2                                                    ]            
where θ=ωt is the angle between the stationary a axis and the synchronous d axis.
Controlling a generator by means of FOC requires a d axis aligned flux component. As explained above, the d and q oriented components are transformations from the stationary three phase coordinate system which implies that the FOC, due to the direct coupling to the three phase electrical quantities, will handle both steady state and transient operation of system irrespective of the generator model.
An advantage of FOC is that the control of parameters in a rotating coordinates theoretically allows uncoupled control between the parameters. Therefore with the amplitude of the rotor flux controlled to at a fixed value and the linear relationship between torque and torque component iq-stator it is possible to achieve satisfactory uncoupled control of WTG output power control
U.S. Pat. No. 5,083,039 discloses a variable speed wind turbine comprising a turbine rotor that drives a multiphase generator, a power converter with switches that control stator electrical quantities in each phase of the generator, a torque command device associated with turbine parameter sensors that generates a torque reference signal indicative of a desired torque, and a generator controller operating under field orientation control and responsive to the torque reference signal for defining a desired quadrature axis current and for controlling the switches to produce stator electrical quantities that correspond to the desired quadrature axis current.
Despite the advantages with FOC disclosed above, there are shortcomings of the conventional controllers that the industry has lived with. These include for example (a) a difficulty maintaining correct decoupling between the flux and torque producing components of the stator currents during steady state and dynamics, (b) controlling the currents using linear controllers at higher speeds and higher modulation index. Case (a) relates to the parameter sensitivity and the need for adaptation of the same. This may put the controller reliability into stress under extreme conditions of load. Case (b) on the other hand relates to under utilization of the DC-link. Further since both of the methods described above may be use for wind turbine output control and output control ultimately involves an interaction between fluxes or currents and fluxes an ability to directly control flux leads to more robust and simpler systems